Among the problems faced in cancer chemotherapy is the development of resistance to treatment regimens. Tumors that respond well to a particular drug or drugs initially, often develop a tolerance to the drug(s). This disease state, called multi-drug resistance, is discussed in greater detail in Kuzmich and Tew, "Detoxification Mechanisms and Tumor Cell Resistance to Anticancer Drugs," particularly section VII "The Multidrug-Resistant Phenotype (MDR)," Medical Research Reviews, Vol. 11, No. 2, 185-217, particularly 208-213 (1991); and in Georges, Sharom and Ling, "Multidrug Resistance and Chemosensitization: Therapeutic Implications for Cancer Chemotherapy," Advances in Pharmacology, Vol. 21, 185-220 (1990).
Certain active agents, called chemosensitizing agents or potentiating agents, have been suggested as resistance modifying agents for treating multidrug resistance, but have suffered from various disadvantageous properties. These have included, e.g., verapamil (a calcium entry blocker that lowers blood pressure and has also been found effective in vitro for treating drug-resistant malaria), steroids, trifluoperazine (a CNS agent), vindoline, and reserpine (an .alpha.-2 blocker with CNS properties). Thus, there has remained a need for active agents to treat, i.e., reverse, inhibit and/or prevent multidrug resistance, preferably with minimal or no adverse side effects.
Chemosensitizing agents interact with P-glycoprotein, a drug efflux pump found in cell membranes, particularly those of multidrug resistant tumor cells, gastrointestinal tract cells, and the endothelial cells that form the blood brain barrier. By blocking this pump, chemosensitizing agents inhibit the efflux of cancer chemotherapeutic drugs from tumor cells, and can enhance permeation of nutrients or active agents through the gastrointestinal tract, and the permeation of active agents through the blood brain barrier.
U.S. Pat. No. 5,112,817 to Fukazawa et al. discloses certain quinoline derivatives useful as anticancer drug potentiators for the treatment of multidrug resistance. One of the initially promising active agents there-disclosed is MS-073, which has the following structure: ##STR2## While highly active in in vitro testing, MS-073 was, however, found to have poor oral bioavailability and to suffer from instability problems in solution. Other compounds of the series, such as the biphenylmethylcarbonyl derivative MS-209, have been found to have better stability and oral bioavailability, but, at the cost of having to administer higher effective doses. Thus, it has remained desired to provide an anticancer drug potentiator having the activity of MS-073, together with good oral bioavailability and stability.